A diaphragm pump typically includes a diaphragm, a pump frame having a holding member in which the diaphragm is held, and a pump head clamping the periphery of the diaphragm onto the pump frame. The diaphragm and the pump head define a pump chamber, the volume of which increases and decreases by the reciprocating action of the diaphragm. A non-return check valve is provided in the intake side and the discharge side of the pump chamber so that a liquid is delivered in one direction.
Because a diaphragm pump has its pump chamber closed and includes no sliding part in the portions in contact with a liquid being handled, there is no fear of debris (that may be produced in a pump having a sliding part, such as a piston pump or a centrifugal pump) or oil getting mixed in the liquid being pumped. Therefore, it is used in handling low or high viscosity liquids, suspensions, corrosive liquid chemicals, and the like in a wide variety of fields, typically in manufacturing products that must be free of foreign matter, such as semiconductors, recording media, and foods.
Although a diaphragm pump permits no foreign matter to enter, it often meets the problem of agglomerates forming on the diaphragm when it handles a liquid containing fine particles that agglomerate easily due to shear stress or collision of the particles, such as a polymer latex.
If agglomerates form on the diaphragm, they can clog the downstream pumping system, such as a check valve. In that case, the diaphragm pump cannot be operated continuously but batchwise such that operation, suspension, and cleaning make one cycle. This impairs the operating efficiency. In the manufacture of products that should be protected from contamination with foreign matter as described, the agglomerates forming on the diaphragm can get mixed in the product and damage the qualities of the product. In the case of handling a coating composition for image formation, for example, the agglomerates will affect coating properties of the coating composition in, e.g., slide coating. Even small agglomerates not so large as to affect slide coating can cause an image defect.
A diaphragm pump for pumping a liquid liable to agglomeration under shear force applied, such as a latex, has been proposed with a structure capable of preventing the liquid from forming an agglomerate as disclosed in JP-U-52-065902.
According to the structure of JP-U-52-065902, a driving rod is attached to a diaphragm to control the reciprocal motion of the diaphragm. The diaphragm is displaced toward the pump head only until it becomes flat so that the range of movement of the diaphragm is only outward of the pump chamber. As a result, the stroke length of the diaphragm is short to reduce the flow rate and to minimize the shear force imposed on the liquid. The diaphragm used in JP-U-52-065902 has its peripheral edge thickened in order to overcome the problem with conventional diaphragms that a liquid sandwiched between the peripheral portion of the diaphragm and the pump head is subjected to excessive shear force to cause particle agglomeration. The increased thickness along the edge of the diaphragm provides an increased gap between the pump head and the peripheral portion of the diaphragm continuous with the thickened edge. As a result, a liquid is prevented from forming agglomerates due to shear force exerted in the gap between the edge of the diaphragm and the pump head.